There is known a high-speed grinding process with a vitrified CBN grindstone having CBN abrasive grains that are held together by a vitrified (inorganic) bonding agent. Such a high-speed grinding process with the vitrified CBN grindstone provides the advantages of reduced wearing amount of the grindstone, prolonged lifetime of the grindstone until the grindstone requires to be dressed, improved efficiency of the grinding operation and improved quality of the ground workpiece. Such a high-speed grinding process has been practiced principally in an outer cylindrical surface grinding operation in which a grindstone having a comparatively small width is generally used, but is recently required to be practiced in also an operation with a centerless grinding apparatus in which a grindstone having a comparatively large axial length is generally used. That is, there is a demand for practice of a high-speed grinding operation at a peripheral speed of the grindstone which is not smaller than about 60m/s, in also an operation with a centerless grinding apparatus. To meet this demand, the used grindstone requires to have a sufficiently high degree of strength for permitting a high-speed revolution thereof, so as to assure a high degree of safety in the grinding operation. Where the grindstone has a mounting hole formed in its center, a maximum stress tends to act on a peripheral edge portion thereof defining the mounting hole. Therefore, a circumferential wall of the mounting hole has to be provided by a material having a sufficiently high degree of braking strength.
In this view, there is proposed a vitrified grindstone including a peripheral edge portion which defines the mounting hole and which is provided by a material having a strength higher than that of a material providing the other portion of the grindstone, so as to permit a higher revolution of the grindstone. Such a proposed grindstone includes a base disk which is made of a steel, aluminum or CFRP (carbon-fiber reinforced plastic), and a single integral annular vitrified abrasive solid mass or a multiplicity of vitrified abrasive segments which is bonded to an outer circumferential surface of the base disk.
However, such a proposed grindstone is difficult to be practically used in an operation since the proposed grindstone tends to have a weight larger than 100Kg where the size of the grindstone is large and the base disk of the grindstone is made of a steel. For permitting the grindstone having such a large weight to be rotated at a high peripheral speed, it is necessary for increasing the power of the grinding apparatus, increasing the rigidity of the axis on which the grindstone is mounted, or even employing a grinding apparatus having a larger degree of overall rigidity.
There are various problems also where the base disk is made of CFRP which is a material suitable for the base disk owing to its light weigh and high strength. For example, it is difficult to provide the base disk with a large thickness, where the base disk is produced by using CFRP as the material, according to a pseudo-isotropic-laminating method. Further, in the interest of minimizing elastic elongation of the base disk and thereby reducing the stress acting on the abrasive layer, CFRP has to have a high degree of elastic modulus, thereby resulting in an increased cost of the production.
There is proposed a base disk having a double structure in which only a radially outer layer is provided by CFRP, as disclosed in JP-A-06-91542. Such a double structure provides various advantages, for example, making it possible to produce the base disk with a reduced amount of CFRP, and to minimize elastic elongation of an outer peripheral portion of the base disk. However, the double structure leads to an increase in the production cost of the base disk, and this increase can not be easily compensated by an increased productivity provided by an increased peripheral speed of the grindstone in a grinding operation. The increase in the production cost of the base disk could be compensated if the grinding operation is performed with a considerably high peripheral speed of the grindstone exceeding 100 m/s, but could not be compensated where the grinding operation is performed with a peripheral speed of the grindstone ranging from 60 to 100 m/s because the grinding operation with the peripheral speed not so high as 100 m/s does not provide a sufficiently increased profit. Further, the abrasive layer or segments can not be stripped from the base disk, by baking the abrasive layer or segment, because the radially outer layer of the base disk is constituted by CFRP. Thus, it is necessary to remove the abrasive layer or segments from the base disk by physically cutting off the abrasive layer or segments, possibly leading to an increased operation cost. In this method, even a portion of the CFRP is undesirably cut off, and the outside diameter of the base disk is accordingly reduced, every time the abrasive layer or segments is removed, resulting in difficulty for repeated reutilization of the base disk. The base disk provides also an environmental disadvantage that the CFRP can not be recycled when the base disk is discarded.
In the above view, there is proposed a base disk which is provided by an aluminum alloy having a modified property. The proposed base disk is produced by compressing and heating aluminum alloy powders and silicone (Si) powders according to a powder metallurgical method, as disclosed in JP-A-07-116963. However, in such a base disk made of the aluminum alloy, the silicone powders are not distributed evenly over the entirety of the base disk due to insufficient dispersion of the silicon powders, resulting in an insufficiently high degree of strength of the base disk. The powder metallurgical method leads to an increased operation cost due to the required compressing and heating processes. A high degree of porosity of the base disk makes it difficult to obtain a high degree of strength of the base disk, making it impossible to increase the thickness of the base disk.